The present invention relates to arrangement for turning an image projected on an image screen, by means of a trapezoidal prism for image turning and an optical system for image producing.
Arrangements of the above mentioned general type are known in the art. One of such arrangements is disclosed for example in U.S. Pat. No. 4,529,276. This arrangement includes an optical system for image for producing and a trapezoidal prism for image turning. A light shield is arranged between the image producing system and the image turning prism which later on will be referred to as prism. The main object of the arrangement in accordance with U.S. Pat. No. 4,529,276 is to reduce the prism without changing the light density distribution of the projected image. For solving this object in this patent the prism is simply shortened. Therefore due to the vignetting of the exit aperture of the projected optical system, the non-uniform distribution of the light density of the projected image exiting the prism is compensated by the light shield. The thusly produced reducing of the prism which directly does not vignet the exit aperture of the image producing system amounts to approximately 50% in volume and approximately 25% in the light path. Since the type of glass suitable for producing of the prism is very expensive and the aberrations of the prism such as for example astigmatism, distortion and color tranverse error are proportional to the light path, there is a requirement due to the high quality requirements with simultaneous cost reduction that the prism be made as small as possible.